In a modern gas turbine engine, components such as blades, vanes, combustor cases and the like are usually made from nickel and cobalt alloys. Nickel and cobalt-based superalloys are most often used to fabricate gas turbine parts because of the high strength required for long periods of service at the high temperatures characteristic of turbine operation. These components are usually located in the "hot section" of the turbine. As such, there are special design requirements imposed upon these components due to the rigorous environment in which they operate. Turbine blades and vanes are often cast with complex hollow core passages for transporting internal cooling air. Also, the wall thickness of gas turbine hot section parts is carefully controlled to balance the need for high temperature strength with the need to minimize the weight of the component part.
The surfaces of turbine engine parts are exposed to the hot gases from the turbine combustion process. Oxidation and corrosion reactions at the surface of the component parts can cause metal wastage and loss of wall thickness. The loss of metal rapidly increases the stresses on the respective component part and can result in part failure. Protective coatings are thus applied to these component parts to protect them from degradation by oxidation and corrosion.
Diffusion aluminide coatings are a standard method for protecting the surfaces of nickel- and cobalt-alloy gas turbine hardware from oxidation and corrosion. Aluminide coatings are based on intermetallic compounds formed when nickel and cobalt react with aluminum at the substrate's surface. An intermetallic compound is an intermediate phase in a binary metallic system, having a characteristic crystal structure enabled by a specific elemental (atomic) ratio between the binary constituents. For example, a number of such phases form in the nickel-aluminum binary system, including Ni.sub.2 Al.sub.3, NiAl, or NiAl.sub.3. Many aluminum-based intermetallic compounds (i.e., aluminides) are resistant to high temperature degradation and therefore are preferred as protective coatings, but such coatings are more brittle than the superalloy substrates underlying the coatings. An example of one particularly useful intermetallic compound formed in nickel-based systems is NiAl.
Careful dimensional tolerances imposed on parts during manufacture must be maintained during the coating process. Uneven or excessively thick diffusion coating layers can effectively act to reduce wall thickness and hence the part's strength. Furthermore, excessively thick aluminide coatings, especially at leading and trailing edges of turbine blades where high stresses mostly occur, can result in fatigue cracking.
One method for applying a diffusion aluminide coating is via a liquid phase slurry aluminization process. Typical slurries incorporate a mixture of aluminum and/or silicon metal powders (pigments) or alloys o those elements in an inorganic binder. The slurries are directly applied to a substrate surface. Formation of the diffused aluminide is accomplished by heating the part in a non-oxidizing atmosphere or vacuum at temperatures between 1600-2000.degree. F. for two to twenty hours. The heating melts the metal in the slurry and permits the reaction and diffusion of the aluminum and/or silicon pigments into the substrate surface. Coatings of this type have been described in U.S. Pat. No. 5,795,659.
In liquid-phase slurry aluminization, the slurry must be applied directly to the part in a controlled amount because the resulting thickness of the diffused coating is directly proportional to the amount of the slurry applied to the surface. Because of this proportional relationship between applied slurry amount and final diffused coating thickness, it is critical in this method to carefully control the application of the slurry material. The necessarily controlled application requires a great deal of operator skill and quality assurance, particularly for parts having complicated geometries such as turbine blades. This places a limit on the quantity of parts that can be coated in an economical, timely fashion.
A more common industrial method for producing aluminide coatings is by the "pack cementation" method. Pack cementation processes have been described, for example, in U.S. Pat. Nos. 3,257,230 and 3,544,348. The basic pack method requires a powder mixture including (a) a metallic source of aluminum, (b) a vaporizable halide activator, usually a metal halide, and (c) an inert filler material such as a metal oxide (i.e., Al.sub.2 O.sub.3).
Parts to be coated with such a mixture are first entirely encased in the pack material and then enclosed in a sealed chamber or "retort". The retort is purged using an inert or reducing gas and heated to a temperature between 1400-2000.degree. F. Under these conditions, the halide activator dissociates, reacts with aluminum from the metallic source, and produces gaseous aluminum halide species. These species migrate to the substrate's surface where the aluminum-rich vapors are reduced by the nickel or cobalt alloy surface to form intermetallic coating compositions.
The amount of aluminum-rich vapors available at the surface of the part is defined by the "activity" of the process. The activity of a process is controlled in general by the amount and type of halide activator, the amount and type of aluminum source alloy, the amount of inert oxide diluent, and the temperature of the process. In some cases other metallic powders such as chromium or nickel are added to influence or "moderate" the aluminum activity in a pack.
The activity of the process influences the structure of the aluminide coating formed. "Low activity" processes produce "outwardly" diffused coatings where the coating forms predominately by the outward migration of nickel from the substrate and its subsequent reaction with aluminum at the part surface. "High activity" processes produce "inwardly" diffused coatings where the coating forms predominately by migration of aluminum into the surface of the substrate.
FIG. 1 shows an outwardly diffused coating structure produced by a low activity process. The original surface of the substrate is labeled. A limitation of outwardly diffused aluminide coatings is that oxides or contaminants present at the original surface of the part can become entrapped within the interior of the final diffused coating structure. If these oxides or contaminants are present in a somewhat continuous manner along the original substrate surface, the effectiveness of the low activity, outwardly diffused coatings is diminished under the stressful operating conditions of the turbine engine.
FIG. 2 shows an example of a higher activity, inwardly-diffused coating structure. The original surface of the substrate is labeled. The aluminum content in the outer zone is sufficient to cause precipitation of elements normally dissolved within the original superalloy substrate. Because of the inward diffusion of aluminum which predominates the coating formation process, oxides and contaminants present at the original substrate surface remain in the outer-most region of the final diffused coating structure where they are less likely to comprise the coating performance.
The pack process generally produces reliably uniform diffused aluminide surface layers on complex shapes such as those characteristic of gas turbine components. However, one major limitation of the pack cementation method is the generation of large amounts of hazardous waste. Considerably more raw material is required in a pack process than a slurry aluminization process. Although the pack mixtures can be "rejuvenated" to some extent with incremental additions of fresh powder, eventually the pack mixture must be replaced and the spent powder disposed in hazardous waste landfills. Dusts from the powder mixture also pose a health risk to employees handling the mixture.
In pack aluminization, the size of the retort, the geometry of the substrate to be coated, and the activity of aluminum in the powder mixture dictate the "ideal" batch size that should be employed to maximize the coating quality. The balance between these factors must be maintained to assure good coating quality, so it becomes difficult to coat batches quickly and cost effectively that are either smaller or larger than the ideal size. Moreover, the speed of the pack process is always slowed by the fact that a retort and a large mass of powder must be heated along with the parts contained therein.
The pack method also limits the speed and cost efficiency of coating production processes because it is essentially a batch process. In a batch process, each operation is completed on every individual part in a group before the next operation commences on any of the parts. In contrast, "one-piece flow" manufacturing is a continuous process which has been shown to be a quick, cost efficient means of production. In continuous coating processes, for example, there is continuous addition to, and withdrawal of, uncoated parts and coated parts from the production system. In "one-piece-flow" processes, an individual component flows directly to a second operation as soon as a first operation is completed, and as another component begins the first operation. Equipment and materials can be grouped so that the flow is balanced to accommodate the different time each operation requires. By non-limiting example only, "one-piece-flow" manufacturing has been widely associated with how the Toyota Corporation (Japan) manufactures automobiles. It is very difficult, and not necessarily economical, to adapt an inherently batch process, like pack aluminizing, to a continuous, one-piece flow manufacture. U.S. Pat. No. 3,903,338 discloses one such attempt.
Improvements in pack aluminide coating processes have also been made by removing the article to be coated from the immediate proximity of the aluminizing powder mixture. U.S. Pat. Nos. 4,132,816 and 4,501,776, for example, describe such aluminizing methods called "above the pack" or "vapor-phase" aluminization processes.
Although a vapor-phase aluminization method is somewhat "cleaner" in that less volume of powder is required, the process is limited to smaller retort volumes, and hence smaller batches of parts can be coated due to the nature of the vapor-phase process. If too large a retort is used, variations in the concentration of vapor-phase reactants occur in regions of the retort, resulting in variations in coating thickness among the parts in the retort. The resultant smaller batch sizes of the vapor-phase method limit production throughput and increase coated part costs.
Vapor-phase aluminization processes tend to operate generally at higher temperatures and lower aluminum activities than pack processes. One consequence of this shift in thermodynamic conditions is a shift in coating structure and composition from a primarily inward, "high activity" growth mechanism (indicative of the pack process) to a primarily outward, "low activity" growth mechanism.
There are other limitations of pack and vapor-phase coating processes. Most gas turbine components have "no coat" areas which must be protected from aluminization during the coating process. For example, most turbine blade root attachments (commonly referred to as "fir trees") must not be coated due to the high fatigue stresses they experience during engine operation. In order to prevent aluminizing vapors from reaching these surfaces during the coating process, one of several masking techniques are usually used.
One method of masking is to apply a layer of metal-rich paste over the "no-coat" regions. The metal-rich layer acts as a "sponge" to absorb the aluminizing vapors. An example of such a metal-rich masking compound is the material "M-7" from Alloy Surfaces (Wilmington, Del.). While the metal-rich paste is effective for the most part in blocking the aluminizing process, it can react with and sinter to the superalloy substrate during the coating process.
For this reason, an intermediate layer of a ceramic-rich paste is usually applied to the part surface prior to application of the metal-rich paste. An example of such a ceramic-rich masking compound is the material "M-1" from Alloy Surfaces (Wilmington, Del.). The ceramic-rich paste has limited blocking ability in a pack or vapor-phase process but it does not react with the part surface and it prevents sintering of the overlayed metal-rich masking paste.
Application of the dual-layer masking compounds is tedious and expensive in coating production processes. In addition, small gaps in the ceramic paste layer may result in the metal-rich paste sintering to the part, forcing the coated part to be scrapped.
A second method of masking, used primarily in vapor-phase processes, is the fabrication of metal masks which are mechanically fastened over the "no-coat" regions. Mechanical masks remove the possibility that undesirable sintering reactions (characteristic of the paste masking method) will occur. However, mechanical masks are part-specific, making them an expensive masking method where multiple part numbers and types are being coated.
Another limitation of pack and vapor-phase coating processes is an attendant heat transfer problem. Many gas turbine components, particularly those fabricated from high-strength cast nickel-base superalloys, require rapid cooling rates when processed at elevated temperatures in order to preserve alloy strength properties. Because of the large mass of pack powder required in pack processes, the necessary cooling rates can not be achieved upon completion of the coating process. This requires that the coated parts receive a second heat treatment after removal from the pack mixture, adding significant additional time and cost to the overall coating operation.
An alternative aluminization process is a vapor-phase slurry aluminization process, that incorporates a halide activator to serve as a source for producing aluminizing vapors (as in the pack aluminization process), but requires direct application of the slurry to the substrate surface. Vapor-phase slurry aluminization requires much less raw material than pack aluminization methods and further eliminates the exposure to dust particulates characteristic of the pack method. Furthermore, since each part has the necessary elements for its diffusion coating applied directly to its surface, there are no batch-size limitations as in pack or vapor-phase aluminization processes.
A limitation of vapor-phase slurry aluminization, however, like the liquid-phase slurry process, is the difficulty in producing a uniform diffused aluminide coating thickness on complex shapes such as turbine air foils. This limitation has prevented halide-activated slurry aluminization from being a viable production process like pack and vapor-phase aluminization for coating entire gas turbine components.
An example of the vapor-phase slurry aluminization process is represented by the material "PWA 545" which is utilized by the aircraft gas turbine industry for local repair of high temperature coatings. This slurry contains a halide activator powder, LiF, along with an aluminum-rich intermetallic compound (Co.sub.2 Al.sub.5) which serves as a source for producing aluminizing vapors. Because of the difficulty in producing uniform diffused aluminide coatings on complex airfoil geometries with this slurry formulation, PWA 545 is not used to aluminize entire turbine blade surfaces, nor is its use permitted on turbine blade leading edges.
European published patent application 0 837 153 A2 to Olsen et al. teaches a method providing a localized aluminide coating using a pack-like mixture. A key feature of EP '153 is that the diffused aluminide coating produced with this method has an outward-type diffusion aluminide microstructure. The EP '153 method utilizes a mixture of an organic binder, a halide activator, a metallic aluminum source, and an inert ceramic material to achieve this particular coating microstructure.
The powder composition described in EP '153 is supplied to a localized region of a part in the form of a tape. The tape is applied to the part in at least one layer, however multiple layers may be employed depending upon the desired thickness of the resulting diffused aluminide. After the tape layer or layers are fixed, the part is then heated to 1800-2000.degree. F. and held for 4 to 7 hours to produce a two-zone, low activity outwardly-diffused aluminide coating. As described in EP '153, the coating produced by this method is formed by nickel from the superalloy slowly diffusing to the surface of the part to combine with aluminum, thereby building up a coating layer of essentially pure NiAl.
Slurry aluminization coating processes are undesirably limited in their application to local regions on a turbine part and are primarily used for spot repair of a damaged pack-produced aluminide coating or vapor-phase aluminide coating. There does not exist in the current art a halide-activated aluminizing slurry formulation which produces reliably uniform diffused aluminide coatings in a uniform manner similar to pack and vapor-phase coating processes.
There is thus a need for a slurry coating composition and a coating method that can aluminize entire air-foil surfaces (regardless of geometry) in a controlled, uniform, repeatable manner thereby overcoming the current limitations of existing slurry aluminization processes. Furthermore, there is a need for a method that utilizes considerably less raw material than the pack method and that minimizes exposure to hazardous materials in the workplace. There is a need for a coating and coating process that minimizes masking requirements for areas of a substrate part that do not require coating. There is a further need for a coating or coating process method that can combine all of these features in a continuous coating process, overcoming the economic limitations of batch processes.